A high-pressure compressor of a turbojet or a turboprop comprises a plurality of compression stages each comprising an annular row of moving blades of a rotor that rotates inside a stationary casing with the blades being mounted on a shaft of the turbomachine, together with an annular row of stator vanes for straightening the air flow, which vanes are carried by the casing via their radially-outer ends.
It is very important for the radial clearances between the moving blades and the casing to be optimized in order to improve the efficiency of the compressor and of the turbomachine, and in order to avoid friction between the ends of the moving blades against the casing which leads to said ends becoming worn and to permanent reduction in the efficiency of the turbomachine at all operating speeds.
Optimizing radial clearances is a problem that is very complex since these clearances depend on different parameters such as operating temperature, which varies from one compression stage to another, speed of rotation of the compressor, and speeds of radial vibration in the stator and the rotor, which speeds are different and also vary from one compression stage to another, because of the different weights and radial dimensions of the moving blades in the various stages.
In order to provide a partial solution to this problem, proposals have already been made to use a double-walled casing comprising a stationary outer wall and an inner wall that is capable of moving radially and that is connected to the outer wall by suspension means that are flexible or deformable, and referred to as “hairpins”.
By calculation, it is possible to determine the radial vibration speeds of the casing and of the rotor at different speeds of operation, and thereafter the shapes, the weights, and the stiffnesses of the hairpins are determined so that the vibrational behavior of the inner wall of the casing is adapted as well as possible to the vibrational behavior of the rotor. By injecting air into the casing, it is also possible to ventilate the hairpins so as to modify their thermal expansion and thereby adjust the radial clearances between the inner wall of the casing and the ends of the moving blades of the various compression stages.
Nevertheless, in the prior art, the compression stages are secured to one another, at least in groups of two, via the inner wall of the casing, which limits the possibilities for adjusting radial clearances because these clearances are adjusted in the same way for the compression stages are connected together, even though the variations in these clearances differ from one stage to another.